Equip your Knife.Then,sneak up on a enemy(Soldier or Dinosaur). CLick your Button to activate the knife attack. Do this very quietly or else you will end up getting detected by your enemy. Remember this can still work on Dinos and Soldiers while their running.

To unlock the Inhuman difficulty, complete the campaign on Normal or Hard. This is how you unlock the Inhuman difficulty on the Xbox 360, I don't know if this is the same for Turok on the PC or the PS3.